Current supply and commutation circuit of electric motor

ABSTRACT

An electronic commutation circuit for a three phase electric motor is supplied with direct current by a circuit having a high internal non-ohmic impedance in relation to the impedance of the load. The supply circuit comprises an electronic interrupter, an inductance coil and a resistance in series with the motor commutation circuit. The supply circuit is opened when the current reaches a selected upper limit value and is closed when the current reaches a lower limit value thereby maintaining the current substantially constant. The limits may be made responsive to a motor function, for example speed or load. The switching of the commutating circuit is effected by transistors or thyristors under control of a cyclical impulse generator and distributor.

United States Patent Favre [4 1 May 8, 1973 [75] Inventor: Robert Favre,Lausanne, Switzerland [73] Assignee: Golay Buchel & Cie S.A., Lausanne,

Switzerland Oct. 26, 1966 Switzerland ..l560l/66 [52] U.S. Cl...318/l38, 318/432, 318/696 [51] Int. Cl. ..II02k 37/00 [58] Field ofSearch ..313/l38, 254, 696,

[56] References Cited UNITED STATES PATENTS 3,495,149 2/1970 Swain..318/l38 TACH GEN.

3,412,307 11/1968 Welch ..3 18/434 X 3,600,658 8/ l 971 Kunijoshi ..318/138 3,414,800 12/1968 Sheldrake et a1. 318/138- X 3,424,962 1/1969Gawron ..3l8/138 3,414,795 12/1968 Weiser... 318/434 X 3,452,263 6/1969Newell ..3 18/696 3,577,176 5/1971 Kreithen.... ..3l8/432 3,526,8199/1970 Graf 318/434 X Primary Examiner-G. R. Simmons AttorneyEmmanuel J.Lobato et al.

[57] ABSTRACT An electronic commutation circuit for a three phaseelectric motor is supplied with direct current by a circuit having ahigh internal non-ohmic impedance in relation to the impedance of theload. The supply circuit comprises an electronic interrupter, aninductance coil and a resistance in series with the motor commutationcircuit. The supply circuit is opened when the current reaches aselected upper limit value and is closed when the current reaches alower limit value thereby maintaining the current substantiallyconstant. The limits may be made responsive to a motor function, forexample speed or load. The switching of the commutating circuit iseffected by transistors or thyristors under control of a cyclicalimpulse generator and distributor.

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IZL v v v v v v CURRENT SUPPLY AND COMMUTATION CIRCUIT OF ELECTRIC MOTORThis application is a continuation-in-part of my application Ser. No.676,871 filed Oct. 20, 1967 now abandoned.

The present invention relates to electronically commutated motors andparticularly to supply and control circuitry for such motors.

The electronic commutation of electric motors either of the inductiontype or synchronous type is well known particularly in its applicationto variable speed motors and especially to motors operating at very highspeeds.

The static invertors utilized for the electronic commutation of electricmotors rarely deliver a sinusoidal output voltage by reason of the veryhigh price of such invertors and the fact that a sinusoidal voltage isnot always best adapted to the purpose. Certain motors, notablysynchronous motors, operate better with a voltage wave that isapproximately rectangular.

There exists a technique known as phase cutting which permits generatingin each phase of the motor a current of optimum wave form. However, theproduction price of equipment utilizing this technique is stillrelatively high.

In a large number of cases a current of approximately rectangular waveform with a regulated amplitude and a limited upper value constitutesthe best compromise between the cost of production of the invertor andthe efficiency of the motor. In accordance with the invention theseobjectives are obtained in a simple manner by:

1. Feeding the motor by a source of direct current having an internalnon-ohmic impedance which is relatively high with respect to the normalimpedance of the load, the output current being regulated, for examplein accordance with the speed or the load of the motor, in such manner asto limit the reactive energy of the motor particularly under low loadconditions.

2. By effecting the cyclical commutation of the phases in such mannerthat the current is always established between two terminals of themotor winding and between two terminals only at any one time.

The characteristics and advantages of the invention will be more fullyunderstood from the following description in conjunction with theaccompanying drawings in which:

FIG. 1 is a circuit diagram of a circuit in which commutation of a threephase motor is effected with semiconductor elements of the thyratrontype;

FIG. 1A shows schematically a cyclical impulse generator controlling thecommutation circuit of FIG.

FIG. 2 is a circuit diagram of a current supply circuit for thecommutated electric motor shown in FIG. 1;

FIG. 3 is a circuit diagram of a commutation circuit using semiconductorelements of the transistor type;

FIG. 4 is a schematic diagram showing the respective vectorial positionsof the phases at the time of transistion of the commutation from onephase to another with the commutation system shown in FIG. 1 as well asin FIG. 3;

FIG. 5 is a graft illustrating the current output characteristics of thecurrent supply circuit shown in FIG. 2.

In FIG. 1 there is shown the commutation circuit of a three phase motorhaving phase windings R, S and T which are shown connected in starconfiguration but can, if desired, be connected in delta configuration,according to whether the motor characteristics of a star configurationor a delta configuration are desired. The commutation circuit comprisesswitching elements in the form of semi-conductors of the thyratron type,i.e. thyristors, for selectively connecting each of the motor windingseither to the negative output line 21 or the positive output line 22 ofa direct current power supply as shown in FIG. 2 and described below.Thus, thyristors 13, I4 and 15 when conductive connect the respectivewindings R, S and T with the positive power supply line 22 whilethyristors 16, 17 and 18 connect the motor windings R, S and Trespectively with the negative power supply line 21. The thyristors 13to 18 are cyclically controlled, as will be described more fully below,so that at any one time one and only one of thyristors 13 to 15 and oneand only one of thyristors 16 to 18 are conductive, the two thyristorsof a pair connected to the same winding never being conductive at thesame time.

Diodes 19 and 20 are connected as shown to absorb reactive currentinduced in the motor windings R, S and T respectively through rotationof the rotor.

There are available thyristors of the turn-off type which can be blockedas well as unblocked by the control electrode. However, thyristors ofthis type are expensive. In the circuit shown in FIG. 1 the thyristors13 to 18 are ordinary thyristors, the blocking of which requires theapplication of a brief pulse of opposite polarity. The blocking pulsesare obtained from auxiliary thyristors 1 to 6 which are also mounted inpairs, 1 and 4, 3 and 6, and 5 and 2. Each pair of auxiliary thyristorscontrol one of the phases R, S or T through a capacitive couplingprovided by capacitors 7, 8 and 9 in series with small dampingresistances 10a, 10b and respectively. Resistances l1 and 12 provideinitial bias. The auxiliary thyristors are capacitively coupled with oneanother as shown so that a single auxiliary thyristor on the positiveside of the circuit and a single auxiliary thyristor on the negativeside of the circuit are conductive at any one time. The thyristors areunblocked in numerical order and the unblocking of a thyristor causesthe blocking of the preceding thyristor by reason of the capacitiveintercoupling.

The auxiliary thyristors of a pair such as 1 and 4 function alternatelyso that it is not necessary to discharge the capacitors 7-9 through anohmic resistance between each blocking operation as is nccessa ry insome circuits.

The firing of the commutation thyristors and auxiliary thyristors iseffected in known manner by pulses supplied, for example by anelectronic impulse counter or a cyclical impulse generator working as acounter and providing pulses sequentially at a predetermined rate. Suchan impulse generator is disclosed in Favre U.S. Pat. No. 3,436,631. Thepulses are for example transmitted to the thyristors inductively by awinding shown between the control electrode and the cathode of eachthyristor in FIG. 1. These windings constitute the secondaries oftransformers, the primaries of which constitute the outputs of thecyclical impulse generator. Thus, in FIG. 1A there is shown by way ofexample an impulse generator 60 having primary output windings P1, P2,P3, P4, P and P6 inductively coupled respectively with secondarywindings S1, S2, S3, S4, S5 and S6 controlling respectively the firingof auxiliary thyristors 1-6. The auxiliary thyristors are fired insequence at a predetermined rate to provide commutation of the motorwindings as described below.

In a given case, the firing of each commutation thyristors issimultaneous with the extinction of another commutation thyristor. Inother words, the firing of one commutation thyristor is simultaneouswith the firing of an auxiliary thyristor, the latter causingsimultaneous blocking of the corresponding commutation thyristor. Hence,the firing of the commutation thyristors can, through suitableconnections, be controlled by the same cyclical impulse generator thatcontrols the auxiliary thyristors. Alternatively, the firing of theauxiliary thyristors and the firing of the commutation thyristors can becontrolled by separate impulse generators properly synchronized with oneanother. The rate at which pulses are cyclically supplied by the impulsegenerator or generators controls the rate of commutation and hence thespeed at which the motor operates.

The sequence of commutation of the motor windings is illustrated by wayof example in FIG. 4 in which the motor windings are represented byvectors R, S and T respectively. The conductivity of the respectivewindings is indicated by heavier lines with arrowheads indicating thedirection of current flow. At the time period represented by diagram a,thyristors 14 and 18 are unblocked so that windings S and T areconducting. Current flows from the positive supply line 22 throughthyristor 14, motor windings S and T and thyristor 18 to the negativesupply line 21. The following switching steps then occur.

The next pulse supplied by the impulse generator 60 through the primarytransformer winding P2 to the corresponding secondary winding S2 causesthe firing of auxiliary thyristor 2 and hence the blocking of thyristors18 and 6 (which have previously been conductive). The blocking ofthyristor 18 causes thyristor 16 to be unblocked. The windings S and Rare-hence I conductive as indicated in diagram b of FIG. 4 so thatcurrent flows from the positive supply line 22 through thyristor 14,motor windings S and R and the thyristor 16 to the negative supply line21.

The next impulse supplied from the primary winding T3 to thecorresponding secondary winding S3 causes the firing of auxiliarythyristor 3. This causes the blocking of the corresponding commutatingthyristor 14 and the auxiliary thyristor l. The blocking of thyristor 14causes the unblocking of thyristor 15. Motor windings T and R are nowconductive as indicated in diagram 0 of FIG. 4. Current flows from thepositive supply line 22 through thyristor 15, motor windings T and R andthyristor 16 to the negative supply line 21.

The next pulse supplied through primary winding T4 to secondary windingS4 causes the firing of auxiliary thyristor 4 and consequently theblocking of commutating thyristor l6 and auxiliary thyristor 2. Theblocking of thyristor 16 causes the unblocking of thyristor 17. Motorwindings T and S are now conductive as illustrated in diagram d of FIG.4 so that current flows from the positive supply line 22 throughthyristor 15, motor windings T and S and thyristor 17 to the negativesupply line 21.

This sequence of operation continues in like manner. Thus, asillustrated in diagram e of FIG. 4, windings R and S are conductingwhile in the following time period represented by diagram f, thewindings R and T are conducting. Commutation of the motor windings isthus effected by way of example in the following sequence:

4-.- Positive alternation S T of phases S R Negative alternationPositive alternation T- R of phase R of phase T T S Negative alternationPositive alternation R S of phase S of phase R R- T Negative alternationS- T of phase T The commutation provided by the commutating thyristors13 to 18 controls not only the speed of the motor but also the directionof rotation of the motor by reason of the sequence in which thethyristors are unblocked. It will be seen that at any one time onewinding and one winding only is connected to the positive supply lineand one winding and onewinding only is connected to the negative supplyline, the winding connected to the negative supply line being, ofcourse, different from that connected to the positive supply line toavoid a short circuit. The wave form of the current supplied by thecommutating thyristors is essentially rectangular. Since each time athyristor is blocked a succeeding thyristor is simultaneously fired, thecurrent drawn from the supply line 21-22 remains essentially constant.

In accordance with the invention current is supplied to the motorcommutation circuit shown in FIG. 1 by a supply circuit having aninternal dynamic impedance which is high relative to the impedance ofthe load so that the current is substantially constant irrespective ofthe load. The ohms loss in the supply circuit is low. Such a circuit isshown by way of example in FIG. 2 in which the circuit of FIG. 1 isrepresented by the block 38 which is shown as in FIG. 1 with inputterminals 21 and 22. The current supply circuit is shown as comprisingtransistors 27, 28, 29 and 30 connected in cascade so that transistor 30is conductive I when transistor 27 is conductive and is nonconductivewhen transistor 27 is blocked. The transistors 27 to 29 are connectedbetween ground and a positive d.c. supply line 39. The emitter oftransistor 30 is connected to ground while the collector is connectedthrough a reactance shown as an induction coil 31 to the terminal 21 ofthe motor commutation circuit 38. The other input terminal 22 of themotor commutation circuit is connected through-a parametric resistance33 to the positive side 40 of a direct current supply. Thus, a flow ofcurrent through the motor commutation circuit and the induction coil 31and measuring resistance 33 connected in series therewith is controlledby the transistor 30.

The conduction of the transistor 27 and hence of transistor 30 iscontrolled by the value of the current flowing through the inductancecoil 31 and hence through the parametric resistance 33. Transistor 27,biased by a resistance 26, is unblocked by the voltage drop across aresistance 24 in the collector circuit of a transistor 23 to the base ofwhich a steady but preferably variable voltage is applied. The voltagedeveloped in the resistance 33 in series with the motor circuit is cutat high frequency by a transistor 34 to the base of which a highfrequency signal is applied by a suitable signal generator and isapplied to the primary 35 of a transformer, the secondary winding ofwhich is connected in series with a diode 25a between the resistance 24and the base of the transistor 27 in such manner that the voltage acrossthe secondary winding 25 rectified by the diode 25a opposes the voltageacross the resistance 24. Hence, when the current through the resistance33 and hence the voltage across this resistance reaches a predeterminedvalue, the resulting voltage transmitted through the primary winding 35to the secondary, winding 25 of the coupling transformer causes blockingof the transistor 27 and hence blocking of the transistor 30.

The operation of the circuit will now be described with reference toFIG. 5 in which the abscissa represents time and the ordinate representsthe value of current supplied to the motor commutation circuit 38.Assuming that at time to the transistor 30 is unblocked so as to connectthe motor commutation circuit 38 in series with the coil 31 andresistance 33 between ground and the positive voltage supply 40, thecurrent in the circuit will increase according to an exponentialfunction, the initial portion of which is essentially linear. When thecurrent has increased to a value I2 the voltage generated in theresistance 33 and applied to the base circuit of the transistor 27causes the transistor 27 and hence transistor 30 to becomenonconductive. By reason of the self-inductance of the coil 31, currentcontinues to be supplied to the circuit 38 through the diode 36. Thiscurrent decreases exponentially until it reaches a lower limit Ilwhereupon the voltage drop across the resistance 33 applied through thetransformer 25, 35 to the base circuit of transistor 27 as decreased toa value at which the transistor 27 and hence the transistor 30 becomeconductive. The current in the circuit 38 thereupon again increasesexponentially until it again'reaches the upper limit.l2. This operationis repeated so that the current oscillates between the lower limit 11and.the upper limit I2 and hence has a mean value Im. The oscillationsare smoothed out by a capacitance 37 connected in shunt with the circuit38 and the resistance 33. Hence, the commutation circuit is continuouslysupplied with current. By reason of the amplification permitted by thecutting operation of transistor 34 and the transformer ratio between theprimary and secondary of the transformer 25, 35 (which is preferably astep-up transformer), the voltage drop in the measuring resistance 33can be limited to a few tenths ofa volt.

The value of the current supplied to the motor circuit 38 can beregulated by varying the voltage applied to the base of the transistor23. This voltage can, if desired, be varied as a function of a parameterof operation of the motor, for example its load or speed. For examplethere is shown in FIG. 2 by way of example a tachometer generator 61which is responsive to motor speed, and hence also to' motor load since(with an induction motor) speed varies with load. The output voltage ofthe tachometer generator is applied to the base of the transistor 23.The voltage of the tachometer generator is applied in such a way thatthe bias voltage of transistor 23 decreases (and hence current limit I2decreases) when the motor speed increases and visa versa. Instead of atach-generator, other known means for producing a voltage that isproportional to the motor speed or the motor load can be employed. Thedifference between the lower limit I1 and the upper limit I2 of thecurrent is determined primarily by the resistance 32.

The frequency of commutation of the transistor 30 depends primarily onthe voltage 40, the voltage across terminals 21-22 (dependent forexample on load conditions of the motor) and on the inductance of thecoil 31. It will be noted that the coil 31 is not traversed by analternating current and in no way opposes the setting up of current inthe motor, a function of rotational speed.

In FIG. 3 there is shown a commutation circuit in which the switching iseffected by transistors rather than thyristors. As in the case of FIG.1, the motor is shown as being of the three phase type with star ordelta connection of the motor windings. The circuit is shown ascomprising six transistors of which transistors 41, 42 and 43 arearranged to connect motor windings R, S and T respectively with thenegative current input line 21 while transistors 44, 45 and 46 arearranged to connect the motor windings respectively with the positivesupply line 22. The current supply lines 21 and 22 are the output linesof the current supply circuit shown in FIG. 2. Thus, the circuit block38 in FIG. 2 represents either the motor commutation circuit of FIG. 1or that of FIG. 3.

The switching of transistors 41 to 43 is controlled by a cyclicalimpulse generator 63 coupled with the transistors through a transformer49 while the switching of transistors 44 to 46 is controlled by acyclical impulse generator 64 coupled with the transistors through atransformer 69. As in the circuit of FIG. 1A, the impulse generatorproduces pulses at a selected frequency and distributes them cyclicallyto the respective switching elements in the manner of an electronicimpulse counter operating in a closed circuit. The

transformer 49 is shown as having primary windings 51, I

53 and 55 with corresponding secondary windings 50, 52 and 54 connectedin the base circuits of transistors 41, 42 and 43 respectively. Only oneof the transistors 41 to 43 can be conducting at one time. If, forexample the primary 51 is energized so as to cause the unblocking oftransistor 41, the secondaries 52 and 54, the respective primaries ofwhich are open, are then subjected to a blocking flux of transistors 42and 43. Diodes 49 avoid an abrupt demagnetization of the correspondingtransformer circuit during the blocking of a transistor. Such abruptdemagnetization would be liable to cause the unwanted simultaneouslyunblocking of two transistors. In like manner, the coupling transformer69 has primaries 71, 73 and connected with the cyclical impulsegenerator 64 while corresponding secondaries 70, 72 and 74 are connectedrespectively in the base circuits of transistors 44, 45 and 46. Diodes66 correspond to the above mentioned diodes 65 and avoid abruptdemagnetization of the corresponding transformer circuits. While twoimpulse generators and two transformers have for convenience been shownin FIG. 3, it will be understood that they may be replaced by a singleimpulse generator and a single coupling transformer having six primariesand six secondaries.

The order of commutation of the motor windings is the same as has beendescribed with reference to FIGS. 1 and 4. As in the case of FIG. 1, oneand only one commutation path is at all times in operation. The resultis a current wave form of substantially rectangular shape owing to thehigh dynamic impedance of the current source shown in FIG. 2. Thecurrent can, as has been described, be made dependent on a function ofthe motor load or speed, for example in such a way as to provide acurrent of minimum value under no low conditions and thereby reduceheating of the motor. Diodes 47 and 48 function like diodes l9 and inFIG. 1 to absorb induced current.

The three phase transformers controlling commutation of the motor mustnaturally transmit useful frequencies from a minimum value which forinduction motor may be around 4 Hz (sliding friction corresponding to amaximum couple at a given current) and a maximum frequency depending onthe speed of the motor. For example the frequency may reach 1,000 Hz fora motor rotating at 60,000 rpm.

The fact that the circuit of FIG. 3 requires only NPN transistorsresults in a relatively low cost price for the circuit while permittingthe use of high voltages and the use of a direct rectification of thevoltage of the three phase sector. Thus, as in the case of FIG. 1, goodoperating characteristics and high efficiency are obtained at a low'cost.

The combination of a commutating circuit as shown in FIGS. 1 and 3 withthe current supply circuit shown in FIG. 2 has the following importantadvantages:

1. The limitation of the maximum current value avoids excess current,for example on starting or through accidental operation of the motor,which might result in the destruction of the commutating semiconductors.

2. The fact that two terminals and two terminals only of the motorwinding are at all times connected with the current input results inapplying to each phase of the motor winding a current ofsubstantiallyrectangular wave form with resulting high torque andefficiency.

3. The possibility of controlling the current as a function of aparameter of the operation of the motor permits limiting the reactiveenergy in the motor to a minimum value thereby increasing the efficiencyof the motor and reducing heating particularly under low loadconditions.

4. The possibility of working in a relatively wide speed range withoutchanging the windings and without a transformer adapter, thesubstantially constant current source automatically adapting the voltageat the terminals of the motor to an optimum value according to the motorspeed.

The circuitry in accordance with the present invention thus providesgood operating characteristics of an electronically commutated motorwhile reducing the cost of production. It will be understood that thisinvention is applicable to electronically commutated motors of both thesynchronous type and the induction type. In either case, the speed ofthe motor is regulated by the impulse generator and distributor whichcontrols the switching elements of the commutating circuit and which ispreferably variable to provide cyclical pulses at selected rates toprovide the motor speed desired. With an induction motor it will beunderstood that slippage occurs between the rotor and the rotating fieldproduced by commutation of the current in the motor windings, theslippage being proportional to'load.

What I claim and desire to secure by Letters Patent 1. A current supplyand commutation circuit of an electric motor having a plurality of phasewindings comprising a motor commutating circuit and a current supplycircuit, said commuting circuit comprising first and second bus lines ofdifferent potential, a pair of switching elements for each motor phasewinding, one of said switching elements of each said pair connecting therespective motor winding with said first bus line and the other of saidswitching elements of each said pair connecting the respective motorwinding with said second bus line, means controlling said switchingelements to operate cyclically in predetermined sequence at a selectedrate to connect one and only one said winding to said first bus line andto connect another and only one of said windings with said second busline at any one time; said supply circuit having output terminalsconnected respectively with said bus lines and comprising a directcurrent power supply line, an electronic interrupter, an inductance coiland a resistor connected in series with said commutation circuit acrosssaid direct current power supply line; a diode connected in shunt acrosssaid series connected inductance coil, said motor commutating circuitand said resistor to provide a path of flow of current from saidreactance coil through said motor commutating circuit when saidinterrupter is open and means sensing the current flowing through saidresistor and controlling said interrupter to open said interrupter whensaid current increases to a selected upper limit and to close saidinterrupter when said current decreases to a selected lower limit,thereby maintaining said current substantially at a selected mean valueirrespective of the impedance of said motor.

2. A circuit. according to claim 1, in which said supply circuitcomprises a plurality of transistors connected in cascade of which afirst transistor constitutes a control transistor and the lasttransistor in said cascade constitutes said interrupter.

3. A circuit according to claim 2, in which said sensing and controllingmeans comprises a high frequency chopper, a transformer having a primarywinding and a secondary winding, and a rectifier, said chopper and saidprimary winding being connected in series with one another across saidresistor and said secondary winding and rectifier being connected incircuit with said first transistor to control the conduction of saidfirst transistor and thereby control said interrupter.

4. A circuit according to claim 2, comprising means for varying saidupper and lower'limits of said current at which said interrupter isopened and closed respectively.

5. A circuit according to claim 4, in which said current limit varyingmeans comprises means for biasing said first transistor and for varyingsaid bias.

6. A circuit according to claim 5, in which said biasing means biasessaid first transistor to conduction and in which said sensing andcontrolling means comprises a high frequency chopper, a transformerhaving a primary winding and a secondary winding and a rectifier, saidchopper and said primary winding being connected in series with oneanother across said resistor, and said secondary winding and rectifierbeing connected in circuit with said first transistor so that therectified output of said secondary winding opposes said bias to blocksaid first transistor when the voltage drop across said resistor assensed by said primary winding and chopper exceeds a selected value.

7. A circuit according to claim 5, in which said biasing means comprisesa bias resistorin the base circuit of said first transistor and a biasregulating transistor controlling the flow of current through said biasresistor.

8. A circuit according to claim 4, in which said means for varying saidupper and lower limits of said current comprises means for varying saidlimits as a function of operation of the motor.

9. A circuit according to claim 8, in which said current limit varyingmeans comprises means for biasing said first transistor and for varyingsaid bias as a func tion of operation of the motor.

10. A circuit according to claim 9, in which said biasing meanscomprises a bias resistor in the base circuit of said first transistor,a bias regulating transistor controlling the flow of current throughsaid bias resistor and means responsive to a function of operation ofthe motor controlling the conduction of said bias regulating transistor.

11. A circuit according to claim 1, comprising a capacitor connected inshunt across said series connected motor commutation circuit andresistor.

12. A circuit according to claim 1, in which said supply circuitcomprises a diode connected in shunt across said series connectedinductance coil, motor commutation circuit and resistor, a plurality oftransistors connected in cascade of which a first transistor constitutesa control transistor and the last transistor of said cascase constitutessaid interrupter, a bias resistor in the base circuit of said firsttransistor, a bias regulating transistor controlling flow of currentthrough said bias resistor to bias said first transistor to conduction;said sensing and controlling means comprising a high frequency chopper,a transformer having a primary winding and a secondary winding and arectifier, said chopper and said primary winding being connected inseries with one another across said first mentioned resistor and saidsecondary winding and rectifier being connected in the base circuit ofsaid first transistor so that the rectified output of said secondarywinding opposes said bias to block said first transistor when thevoltage drop across said first mentioned resistor as sensed by saidprimary winding and chopper exceeds a selected value.

1. A current supply and commutation circuit of an electric motor havinga plurality of phase windings comprising a motor commutating circuit anda current supply circuit, said commuting circuit comprising first andsecond bus lines of different potential, a pair of switching elementsfor each motor phase winding, one of said switching elements of eachsaid pair connecting the respective motor winding with said first busline and the other of said switching elements of each said pairconnecting the respective motor winding with said second bus line, meanscontrolling said switching elements to operate cyclically inpredetermined sequence at a selected rate to connect one and only onesaid winding to said first bus line and to connect another and only oneof said windings with said second bus line at any one time; said supplycircuit having output terminals connected respectively with said buslines and comprising a direct current power supply line, an electronicinterrupter, an inductance coil and a resistor connected in series withsaid commutation circuit across said direct current power supply line; adiode connected in shunt across said series connected inductance coil,said motor commutating circuit and said resistor to provide a path offlow of current from said reactance coil through said motor commutatingcircuit when said interrupter is open and means sensing the currentflowing through said resistor and controlling said interrupter to opensaid interrupter when said current increases to a selected upper limitand to close said interrupter when said current decreases to a selectedlower limit, thereby maintaining said current substantially at aselected mean value irrespective of the impedance of said motor.
 2. Acircuit according to claim 1, in which said supply circuit comprises aplurality of transistors connected in cascade of which a firsttransistor constitutes a control transistor and the last transistor insaid cascade constitutes said interrupter.
 3. A circuit according toclaim 2, in which said sensing and controlling means comprises a highfrequency chopper, a transformer having a primary winding and asecondary winding, and a rectifier, said chopper and said primarywinding being connected in series with one another across said resistorand said secondary winding and rectifier being connected in circuit withsaid first transistor to control the conduction of said first transistorand thereby control said interrupter.
 4. A circuit according tO claim 2,comprising means for varying said upper and lower limits of said currentat which said interrupter is opened and closed respectively.
 5. Acircuit according to claim 4, in which said current limit varying meanscomprises means for biasing said first transistor and for varying saidbias.
 6. A circuit according to claim 5, in which said biasing meansbiases said first transistor to conduction and in which said sensing andcontrolling means comprises a high frequency chopper, a transformerhaving a primary winding and a secondary winding and a rectifier, saidchopper and said primary winding being connected in series with oneanother across said resistor, and said secondary winding and rectifierbeing connected in circuit with said first transistor so that therectified output of said secondary winding opposes said bias to blocksaid first transistor when the voltage drop across said resistor assensed by said primary winding and chopper exceeds a selected value. 7.A circuit according to claim 5, in which said biasing means comprises abias resistor in the base circuit of said first transistor and a biasregulating transistor controlling the flow of current through said biasresistor.
 8. A circuit according to claim 4, in which said means forvarying said upper and lower limits of said current comprises means forvarying said limits as a function of operation of the motor.
 9. Acircuit according to claim 8, in which said current limit varying meanscomprises means for biasing said first transistor and for varying saidbias as a function of operation of the motor.
 10. A circuit according toclaim 9, in which said biasing means comprises a bias resistor in thebase circuit of said first transistor, a bias regulating transistorcontrolling the flow of current through said bias resistor and meansresponsive to a function of operation of the motor controlling theconduction of said bias regulating transistor.
 11. A circuit accordingto claim 1, comprising a capacitor connected in shunt across said seriesconnected motor commutation circuit and resistor.
 12. A circuitaccording to claim 1, in which said supply circuit comprises a diodeconnected in shunt across said series connected inductance coil, motorcommutation circuit and resistor, a plurality of transistors connectedin cascade of which a first transistor constitutes a control transistorand the last transistor of said cascase constitutes said interrupter, abias resistor in the base circuit of said first transistor, a biasregulating transistor controlling flow of current through said biasresistor to bias said first transistor to conduction; said sensing andcontrolling means comprising a high frequency chopper, a transformerhaving a primary winding and a secondary winding and a rectifier, saidchopper and said primary winding being connected in series with oneanother across said first mentioned resistor and said secondary windingand rectifier being connected in the base circuit of said firsttransistor so that the rectified output of said secondary windingopposes said bias to block said first transistor when the voltage dropacross said first mentioned resistor as sensed by said primary windingand chopper exceeds a selected value.